In an extract from her candid and controversial new memoir, Rachel Cusk describes the breakdown of her marriage and its painful aftermath

Recently my husband and I separated, and over the course of a few weeks the life that we'd made broke apart, like a jigsaw dismantled into a heap of broken-edged pieces.

'The new reality' was a phrase that kept coming up in those early weeks: people used it to describe my situation, as though it might represent a kind of progress. But it was in fact a regression: the gears of life had gone into reverse. The new reality, as far as I could see, was only something broken. It had been created and for years it had served its purpose, but in pieces – unless they could be glued back together – it was good for nothing at all.

My husband said that he wanted half of everything, including the children.

No, I said.

What do you mean no, he said.

You can't divide people in half, I said.

They should be with me half the time, he said.

They're my children, I said. They belong to me.

Once I would have criticised such a sentiment severely, but of certain parts of life there can be no foreknowledge. Where had this heresy gestated? If it was part of me, where had it lived for all those years, in our egalitarian household? Where had it hidden itself?

Call yourself a feminist, my husband says. And perhaps one of these days I'll say to him, yes, you're right. I shouldn't call myself a feminist. You're right. I'm so terribly sorry.

And in a way, I'll mean it. What is a feminist, anyway? What does it mean, to call yourself one? There are men who call themselves feminists. There are women who are anti-feminist. A feminist man is a bit like a vegetarian: it's the humanitarian principle he's defending, I suppose. Sometimes feminism seems to involve so much criticism of female modes of being that you could be forgiven for thinking that a feminist is a woman who hates women, hates them for being such saps. Then again, the feminist is supposed to hate men. She is said to scorn the physical and emotional servitude they exact. Apparently she calls them the enemy.

In any case, she wouldn't be found haunting the scene of the crime, as it were; loitering in the kitchen, in the maternity ward, at the school gate. She knows that her womanhood is a fraud, manufactured by others for their own convenience; she knows that women are not born but made. So she stays away from it, the kitchen, the maternity ward, like the alcoholic stays away from the bottle. Some alcoholics have a fantasy of modest social drinking: they just haven't been through enough cycles of failure yet.

The woman who thinks she can choose femininity, can toy with it like the social drinker toys with wine – well, she's asking for it, asking to be undone, devoured, asking to spend her life perpetrating a new fraud, manufacturing a new fake identity, only this time it's her equality that's fake. Either she's doing twice as much as she did before, or she sacrifices her equality and does less than she should. She's two women, or she's half a woman. And either way she'll have to say, because she chose it, that she's enjoying herself.

So I suppose a feminist shouldn't get married. She shouldn't have a joint bank account or a house in joint names. Perhaps she shouldn't have children either, girl children whose surname is not their mother's but their father's, so that when she travels abroad with them they have to swear to the man at passport control that she is their mother. No, I shouldn't have called myself a feminist, because what I said didn't match with what I was.

For me, to act as a mother, I had to suspend my own character. I was aware, in those early days of motherhood, that my behaviour was strange to the people who knew me well. It was as though I had been brainwashed, taken over by a cult religion.

And yet this cult, motherhood, was not a place where I could actually live. Like any cult, it demanded a complete surrender of identity to belong to it. So for a while I didn't belong anywhere. As the mother of young children I was homeless, drifting, itinerant. And I felt an inadmissible pity for myself and for my daughters in those years. It seemed, almost, catastrophic to me, the disenchantment of this contact with womanhood.

And so I conscripted my husband into care of the children. My notion was that we would live together as two hybrids, each of us half male and half female. He gave up his law job, and I gave up the exclusivity of my primitive maternal right over the children. These were our preparatory sacrifices to the new gods, whose future protection we hoped to live under. Ten years later, sitting in a solicitor's office on a noisy main road in north London, my maternalism did indeed seem primitive to me, almost barbaric.

The children belong to me – this was not the kind of rudimentary phrase-making I generally went in for. Yet it was the only thought in my head, there in the chrome and glass office, with the petite solicitor in tailored black sitting opposite. She told me I was obliged to support my husband financially, possibly for ever. But he's a qualified lawyer, I said. And I'm just a writer. What I meant was, he's a man. And I'm just a woman.

What I need is a wife, jokes the stressed-out feminist career woman, and everyone laughs. The joke is that the feminist's pursuit of male values has led her to the threshold of female exploitation. I had hated my husband's unwaged domesticity just as much as I had hated my mother's; and he, like her, had claimed to be contented with his lot.

Why had I hated it so? Because it represented dependence. But there was more to it than that, for it might be said that dependence is an agreement between two people. My father depended on my mother too: he couldn't cook a meal, or look after children from the office. They were two halves that made up a whole. What, morally speaking, is half a person? Yet the two halves were not the same: in a sense my parents were a single compartmentalised human being. My father's half was very different from my mother's, but despite the difference neither half made any sense on its own. So it was in the difference that the problem lay.

My notion of half was more like the earthworm's: you cut it in two, but each half remains an earthworm, wriggling and fending for itself. I earned the money in our household, did my share of the cooking and cleaning, paid someone to look after the children while I worked, picked them up from school once they were older. And my husband helped. It was his phrase, and still is: he helped me. I was the compartmentalised modern woman, the woman having it all, and he helped me to be it, to have it. But I didn't want help: I wanted equality. In fact, this idea of help began to annoy me.

Why couldn't we be the same? Why couldn't he be compartmentalised too? And why, exactly, was it helpful for a man to look after his own children, or cook the food that he himself would eat? Helpful is what a good child is to its mother. A helpful person is someone who performs duties outside their own sphere of responsibility, out of the kindness of their heart.

Help is dangerous because it exists outside the human economy: the only payment for help is gratitude. And did I not have something of the same gratuitous tone where my wage-earning was concerned? Did I not think there was something awfully helpful about me, a woman, supporting my own family?

And so I felt, beneath the reconfigured surface of things, the tension of the old orthodoxies. We were a man and a woman who in our struggle for equality had simply changed clothes. We were two transvestites, a transvestite couple – well, why not? Except that I did both things, was both man and woman, while my husband – meaning well – only did one. Once, a female friend confessed to me that she admired our life but couldn't have lived it herself. She admitted the reason – that she would no longer respect her husband if he became a wife.

Sometimes, in the bath, the children cry. Their nakedness, or the warm water, or the comfort of the old routine – something, anyway, dislodges their sticking-plaster emotions and shows the wound beneath. It is my belief that I gave them that wound, so now I must take all the blame.

When my children cry a sword is run through my heart. Yet it is I who am also the cause of their crying. And for a while I am undone by this contradiction, by the difficulty of connecting the person who acted out of self-interest with the heartbroken mother who has succeeded her.

We rearrange the furniture to cover up the gaps. We economise, take in a lodger, get a fishtank. The fish twirl and pirouette eternally amidst the fronds, regardless of what day it is. The children go to their father's and come back again. They no longer cry: they complain heartily about the inconvenience of the new arrangements. A friend comes to stay and remarks on the sound of laughter in the house, like birdsong after the silence of winter.

But it is winter still: we go to a Christmas carol service and I watch the other families. I watch mother and father and children. And I see it so clearly, as though I were looking in at them through a brightly lit window from the darkness outside; see the story in which they play their roles, their parts, with the whole world as a backdrop.

We're not part of that story any more, my children and I. We sing the carols, a band of three. I have sung these songs since my earliest recollection, sung them year after year: first as a tradition-loving child in the six-strong conventional family pew; later as a young woman who most ardently called herself a feminist; later still as a wife and mother in whose life these unreconcilable principles – the traditional and the radical, the story and the truth – had out of their hostility hatched a kind of cancer. Looking at the other families I feel our stigma: we are like a gypsy caravan parked up among the houses, itinerant, temporary.

I begin to notice, looking in through those imaginary brightly lit windows, that the people inside are looking out. I see the women, these wives and mothers, looking out. They seem happy enough, contented enough, capable enough: they are well dressed, attractive, standing with their men and their children. Yet they look around, their mouths moving.

It is as though they are missing something or wondering about something. I remember it so well, what it was to be one of them. Sometimes one of these glances will pass over me and our eyes will briefly meet. And I realise she can't see me, this woman whose eyes have locked with mine. It isn't that she doesn't want to, or is trying not to. It's just that inside it's so bright and outside it's so dark, and so she can't see out, can't see anything at all.